Whatever the small signal model of this is doing, IDK yet, but very similar circuits I've worked on and simulated, pretty much just actted as a comparator for a large signal.
So like for a computer PSU, if the 12V rail DC value went to 12.3V, the opto-coupler would turn on, and it would pull down it's output voltage from all the way off=logic high, to all the way on=logic low on the collector and some pull-up resistor. Then the controller IC see's that, and shut's off the switching, on the primary side, or slows it down, etc.
But the 3-termainal adjustable regular, is basically a settable zener, to use as a nearly fixed voltage, or current sink, and then choose some of the resistors such that the LED part of the opto, is not on, they need about 1V to be running. And then when the Vin voltage gets a bit too high, the LED turns on, and opens the collector-emitter path, and the collector current pulls down some resistor voltage for a chip to see (that's it's not what it should be).
For AC signals, there's the path through the cap's and other capacitiances that come into play, to get the system response needed, to cope with load changes. I've been too busy or lazy to learn the AC models for these yet tho, but they are just regular inverting amplifer and BJT circuits for the most parts I guess.
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